OVERALL SUMMARY Malaria in the Greater Mekong Subregion (GMS) of Southeast Asia remains an important public health problem. The central goal of the ICEMR program is to improve our understanding of how mobile human populations, parasite drug resistance, and mosquito biology contribute to continuous malaria transmission at international borders so that innovative control strategies can be developed to support regional malaria elimination. To achieve this overarching objective, we have selected study sites in the international border regions of three GMS countries, Myanmar, China, and Thailand, with drastically different malaria epidemiology to conduct comprehensive research on humans, vectors, and parasites in four interrelated projects. While these projects have been continuously generating important findings on the factors contributing to persistent malaria transmission, regional needs for implementing public health interventions targeting the residual Plasmodium vivax transmission are high. The Ministries of Public Health in Thailand and Myanmar are considering integrating mass primaquine preventive treatment (MPPT) into their national malaria control programs, but are hesitating because of a lack of clear evidence for the effectiveness of this strategy for eliminating vivax malaria in tropical countries. Therefore, the overall goal of this supplement is to critically assess the effectiveness, feasibility and sustainability of MPPT in the low-endemicity, P. vivax-predominant areas of Thailand and Myanmar. In the Observational Studies, we aim to assess the acceptability, feasibility and readiness of conducting MPPT in malaria-endemic communities and to assess public awareness, readiness and ability of existing healthcare facilities for malaria elimination. In the Implementation Studies, we will use a two-phase approach to conduct cluster-randomized trials to evaluate safety, effectiveness, and feasibility of MPPT in small (year 1-2) and large (year 3-5) malaria communities in Thailand and Myanmar. By actively engaging the public sectors at both local and central levels, we will provide scientifically rigorous data to inform policy makers. Collectively, this ICEMR program, built on the scientific achievements of the current ICEMR program and the strong network of international collaborators, will make even greater impacts on propelling the regional malaria elimination campaign.